Do the circulating neurohypophysial hormones affect basal or stress induced prolactin (PRL) release in male rats?

The action of acute administration of oxytocin (OXY), vasopressin (AVP) or its analog 1-deamino-8-D-arginine-vasopressin (dDAVP) on basal and stress induced PRL release in normal male rats and the effect of chronic injection of AVP on PRL stress response in AVP deficient rats were studied. The hormones (OXY, 600 ng min-1 per rat; AVP 6, 12 or 24 ng min-1 per rat and dDAVP 24 ng min-1 per rat) were infused to conscious rats via the jugular vein for 10 min and then the rats were immobilized under continuing the infusion for further 20 min. In parallel experiments arterial blood pressure (BP) was measured. OXY and 24 ng min-1 AVP caused high BP elevation of the same magnitude, yet the effect of 12 ng min-1 AVP was significantly lower. Neither OXY, dDAVP, nor 6 and 12 ng min-1 of AVP affected basal or stress stimulated PRL values when compared with saline treated animals. 24 ng min-1 of AVP highly stimulated nonstressed PRL levels and no additional stress effect was observed. Intramuscular injection of 2 micrograms (1 U) of AVP daily for 7 days did not influence the basal values or stress induced PRL response in Brattleboro homogygous rats as compared with vehicle treated controls or heterozygous rats treated with AVP or vehicle. These results show that the infusion of 24 ng min-1 per rat of AVP stimulated PRL release which cannot be explained by the nonspecific effect of high BP. Repeated AVP administration did not modulate either the basal or IMO stress stimulated PRL secretion in rats with or without genetic vasopressin deficiency.